The operating temperature is one of the most important factors affecting the reliability and performance of semiconductor chips. Large scale integration of integrated circuits leads to a higher power density, and means more and more heat dissipation per surface area of the semiconductor chip. Materials, packages, processing and cooling methods of semiconductor chips have to meet strong requirements to keep the operating temperature below a critical junction temperature specified by the manufacturer and which must not be exceeded or failure of the semiconductor chip will occur. Especially high technology and power devices must effectively transfer heat to the ambient.
Thermal performance of packaged semiconductor chips is traditionally expressed by the steady-state junction-to-case thermal resistance between junction and some reference location at the package. The thermal resistance is defined as R.sub.n =(T.sub.j -T.sub.ref)/P, wherein T.sub.j denotes the highest chip temperature further referred to as the junction temperature, T.sub.ref is some reference temperature at the package, and P denotes the electrical power being dissipated in the semiconductor chip. Standard measurement procedures such as SEMI G30-88, SEMI G43-87, MIL-STD-883C, Method 1012 should maintain consistent and repetitive results. These methods offer a choice of using either a fluid bath or a heat-sink environment and both assume that the measured thermal resistance is independent of such environmental choice. Unfortunately, the environmental conditions (e.g. flow field in fluid bath, circulation rate) may strongly influence the measured junction-to-case thermal resistance. As a result, strong differences in thermal resistance can be measured when using the same standard method in different environmental conditions. Moreover, according to the mentioned standards, the reference temperature is measured using a thermocouple in contact with the package surface. Attachment of a thermocouple heavily disturbs the heat transfer between package and environment. Large system disturbance errors are thus introduced in the measurement chain, resulting in huge uncertainties of the actual thermal resistance value.
As the reliability of semiconductor chips does not only depend on the steady-state temperature, but also on temperature changes and gradients, measurement of the transient thermal behaviour of semiconductor chips has gained increasing importance. The thermal impedance, defined as the transient thermal resistance, contains a lot of information concerning the semiconductor chip integrity and chip attach quality. Thermal impedance measurement techniques are described in MIL-STD-750C for various devices, such as diodes and transistors. The environmental conditions are not specified, though it is assumed that the package reference location is kept at a constant temperature.
There are disclosed in "Journal de la Physique, Part 3, Number 3, April 1993, Paris, of Pierre Paris, Jean-Marie Hausonne et Jean Lostec, for "Appareil permettant la caracterisation thermique de substrats et materiaux a forte conductibilite pour micro-electronique" an apparatus and method for characterising the thermal conductivity of substrates used in semiconductor packaging. This method is limited to measure the thermal properties of ceramic substrate samples and cannot be applied to packaged semiconductor chips.
U.S. Pat. No. 4,840,495 describes a method and apparatus for measuring the thermal resistance of a chip assembly including a semiconductor chip which is glued on a substrate. This method requires the semiconductor chip be accessible at the heat source side. Though, in a lot of applications a semiconductor chip is enclosed in some type of package which prevents a direct contact at the chip heat source. Thus it is not possible, with the methods of the prior art, to measure the thermal resistance of fully packaged semiconductor chips.
Due to the inaccessibility of the packaged semiconductor chip, an indirect but non destructive method is used to determine the junction temperature, which is defined as highest temperature. Temperature sensitive electrical parameters on the chip can be used as thermometers for indirect sensing of chip temperature.